Boron oxide-silicon dioxide mixed oxide

ABSTRACT

A boron oxide-silicon dioxide mixed oxide which has a BET surface of less than 100 m 2  g, and optionally containing oxides of aluminium, titanium or zirconium, is prepared pyrogenically by flame hydrolysis. The mixed oxide is used in glass making.

This application is based on application no. 19624392.0 filed in Germany on Jun. 19, 1996 and provisional application No. 60/029,845 filed in the United States on Oct. 29, 1996, the content of which are incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a boron oxide-silicon dioxide mixed oxide, a process for its preparation, and its use.

2. Prior Art

Document DE-A 21 22 066 describes the use or pyrogenically produced mixed oxides of boron with silicon, aluminum, titanium and/or iron having a boron content of 2 to 20 wt. %, calculated as elementary boron, as a filler in organopolysiloxane compositions (for example bouncing putty). In particular these mixed oxides contain 5 to 10 wt. % of boron, calculated as elementary boron. These mixed oxides can be prepared by using metals and/or metal compounds according to Ullmanns Enzyclopadie der technischen Chemie, (Ullmanns Encyclopedia of Industrial Chemistry), Vol. 15 (1964), p.726. They are preferably prepared from volatile compounds of boron and silicon, aluminum, titanium and/or iron, especially the chlorides of the aforementioned elements, in the presence of water formed in situ at temperatures above 800° C., i.e. by flame hydrolysis. The mixed oxides obtained have a BET surface of 100 to 400 m² /g.

According to the example given in document DE-A 21 22 066, 4.8 kg/h of SiCl₄ and 1.2 kg/h of boron trichloride are vaporised and burnt together with 1.5 m³ /h of hydrogen in a combustion chamber, under the addition of 4.2 m³ /h of air. The temperature in the combustion chamber is more than 800° C. The resultant mixed oxide contains 82.5 wt. % of SiO2 and 17.5 wt. % of B₂ 03 (=5 wt. % of boron) calculated as boron and has a BET surface of 180 m² /g.

The known boron oxide-silicon dioxide mixed oxide has the disadvantage that it occurs in very finely divided form having a BET surface greater than 100 m² /g. It is unsuitable for use as a raw material in glass-making.

The known process has the disadvantage that boron trichloride is used as starting material for the boron oxide. Boron trichloride is a poisonous compound with a boiling point of 12.5° C. When used at room temperature special safety measures are therefore necessary. The commercially available boron trichloride may contain noticeable amounts of poisonous phosgene as impurity.

The object of the invention is therefore to prepare a boron oxide-silicon dioxide mixed oxide that does not have these disadvantages.

SUMMARY OF THE INVENTION

The invention provides a boron oxide-silicon dioxide mixed oxide, which is characterised in that it has a BET surface of less than 100 m² /g, preferably 10 to 80 m² /g, in particular 25 to 50 m² /g.

The proportion of boron oxide may be from 0.01 to 40 wt. %.

In a further development of the invention the boron oxide-silicon dioxide mixed oxide may contain a further oxide or further oxides of metals and/or metalloids as a constituent of the mixed oxide.

Such metals or metalloids may be aluminum, titanium or zirconium.

In a preferred embodiment the boron oxide-silicon dioxide mixed oxides according to the invention may be prepared pyrogenically.

The known process of flame hydrolysis may preferably be employed for this purpose. This process is described in Ullmanns Enzyclopadie der technischen Chemie, 4th Ed., Vol. 21 (1982), pp. 464 and 465.

The boron oxide-silicon dioxide mixed oxide according to the invention having a BET surface of less than 100 m² /g, preferably 10 to 80 mg² /g, in particular 25 to 50 m² /g, can be prepared by vaporising silicon halides and/or organosilicon halides, for example methyl trichlorosilane, preferably the chloride SiCl₄ with a vaporisable boron compound, for example trimethyl borate, separately or together, optionally with the addition of vaporisable compounds of the metalloids and/or metals aluminum, titanium, zirconium, for example AlCl₃, ZrCl₄, TiCl₄ or the like, mixing the vapours together with a carrier gas, for example air and/or nitrogen, in a mixer unit, preferably in a burner of known construction, with hydrogen as well as air and optionally further gases such as oxygen and nitrogen, reacting the gases in a flame, then cooling the hot gases and the solid, separating the gases from the solid, and optionally removing halide or raw material residues adhering to the product by a heat treatment with moist air.

The process according to the invention has the advantage that the trimethyl borate that is used is miscible in all proportions with SiCl₄, and it is therefore possible to adjust the ratio exactly. Since the boiling point of trimethyl borate of 68° C. is sufficiently close to the boiling point of silicon tetrachloride, it can be vaporised jointly with silicon tetrachloride from a vaporisation unit.

The boron oxide-silicon dioxide mixed oxide according to the invention, which may optionally also contain TiO₂, Al₂ O₃ and/or ZrO₂, can be used to make high-purity glasses. The high purity and the adjusted particle fineness of the mixed oxide according to the invention are of particular advantage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention now will be described with respect to the accompanying drawings wherein:

FIG. 1 is a diagrammatic illustration of a burner arrangement used for Examples 1-4 of the invention;

FIG. 2 is a diagrammatic illustration of a burner arrangement used for Examples 5-9 of the invention;

FIG. 3 is an electron microscope photograph of the powder of Example 2;

FIG. 4 is an electron microscope photograph of the powder of Example 9;

FIG. 5 is a summation curve of the number distribution of Example 2;

FIG. 6 is a differential curve of the number distribution of Example 2;

FIG. 7 is a class frequency representation of the number distribution of Example 2;

FIG. 8 is a summation curve of the weight distribution of Example 2;

FIG. 9 is a differential curve of the weight distribution of Example 2;

FIG. 10 is a class frequency representation of the weight distribution of Example 2;

FIG. 11 is a summation curve of the number distribution of Example 9;

FIG. 12 is a differential curve of the number distribution of Example 9;

FIG. 13 is a class frequency representation of the number distribution of Example 9;

FIG. 14 is a summation curve of the weight distribution of Example 9;

FIG. 15 is a differential curve of the weight distribution of Example 9; and

FIG. 16 is a class frequency representation of the weight distribution of Example 9.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION EXAMPLES Preparation of Boron Oxide-Silicon Dioxide Mixed Oxide Examples 1 to 4

The burner arrangement for examples 1 to 4 is shown diagrammatically in FIG. 1.

The burner is housed in a heated furnace part, the temperature of the furnace being roughly the same as the outflow temperature of the gas mixture. The burner consists of a mixing chamber to which is connected a nozzle from which the gas mixture flows, as well as a second (jacket) nozzle annularly surrounding the first nozzle, from which in addition (jacket) hydrogen is blowN into the flame in order to avoid agglomerations (in the nozzle region).

Example 1

1.0796 kg/h of SiCl4 are vaporised at ca. 130° C. and conveyed together with 0.1 Nm³ /h of nitrogen as carrier gas into the mixing chamber of the burner. 0.51 Nm³ /h of air as well as 0.107 Nm³ /h of oxygen and 0.3 Nm³ /h of (core) hydrogen are ) fed into the same mixing chamber.

0.0643 kg/h of trimethyl borate are vaporised at ca. 120° C. in a separate vaporisation device and are likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm³ /h). The gas mixture flows at a rate of 12 m/sec (referred to standard conditions) from the nozzle opening of the burner and burns in a reaction chamber (flame pipe).

The temperature of the gas mixture (SiCl₄)-air-hydrogen, oxygen, trimethyl borate) is measured at the mouth of the burner, and is found to be 128° C. 0.05 Nm³ /h of hydrogen (jacket hydrogen, room temperature) are fed into the annular nozzle surrounding the mouth of the burner.

After the flame hydrolysis the reaction gases and the boron oxide-silicon dioxide mixed oxide are sucked through a cooling system by applying a vacuum and are cooled to ca. 100° to 160° C. The solid is separated from the waste gas stream in a filter. The boron oxide-silicon dioxide mixed oxide occurs as a white, finely particulate powder.

The specific surface is 45 m² /g, and the analytically determined B₂ 0₃ content is 4.2 wt. % (=1.3 wt. % of boron, calculated as boron).

The adjustment parameters are summarised in Table 1.

Example 2

The procedure described in Example 1 is employed and the following amounts are used:

0.9387 kg/h of SiCl₄ with 0.1 Nm³ /h of nitrogen as carrier gas, air 0.76 Nm³ /h, oxygen 0.159 Nm³ /h (core) hydrogen 0.268 Nm³ /h.

0.05 Nm³ /h of hydrogen are fed into the annular nozzle surrounding the mouth of the burner. The gas mixture flows at a rate of 14 m/sec (referred to standard conditions) from the nozzle opening of the burner.

0.1777 kg/h of trimethyl borate are vaporized in a separate vaporization device and likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm³ /h).

The specific surface of the boron oxide-silicon dioxide mixed oxide is 34 m² /g, and the analytically determined B₂ 0₃ content is 13.3 wt. % (=4.1 wt. % of boron, calculated as boron).

The adjustment parameters are summarised again in Table 1.

The boron oxide-silicon dioxide obtained has the following particle size distribution:

    ______________________________________                                         Total number of particles                                                                            (N) 3223.                                                Particle diameter, arithmetic mean                                                                   (DN) 45.72 (NM)                                          Particle diameter, averaged over                                                                     (DA) 60.81 (NM)                                          surface                                                                        ______________________________________                                    

Percentage Distribution

    ______________________________________                                                                      Sum    Sum                                        Diameter  Number  Number     Number %                                                                              Wt. %                                      D (NM)    N       %          SN %   SND %                                      ______________________________________                                         7.450     40.     1.241      1.241  0.003                                      10.210    123.    3.816      5.057  0.030                                      12.970    155.    4.809      9.867  0.099                                      15.730    105.    3.258      13.124 0.182                                      18.490    56.     1.738      14.862 0.254                                      21.250    64.     1.986      16.848 0.380                                      24.010    64.     1.986      18.833 0.560                                      26.770    78.     2.420      21.253 0.865                                      29.530    98.     3.041      24.294 1.379                                      32.290    86.     2.668      26.962 1.970                                      35.050    109.    3.382      30.344 2.926                                      37.810    127.    3.940      34.285 4.325                                      40.570    173.    5.368      39.652 6.680                                      43.330    198.    6.143      45.796 9.963                                      46.090    193.    5.988      51.784 13.814                                     48.850    191.    5.926      57.710 18.352                                     51.610    178.    5.523      63.233 23.339                                     54.370    160.    4.964      68.197 28.580                                     57.130    153.    4.747      72.944 34.395                                     59.890    135.    4.189      77.133 40.306                                     62.650    140.    4.344      81.477 47.322                                     65.410    113.    3.506      84.983 53.767                                     68.170    109.    3.382      88.365 60.805                                     70.930    86.     2.668      91.033 67.060                                     73.690    64.     1.986      93.019 72.280                                     76.450    52.     1.613      94.632 77.015                                     79.210    43.     1.334      95.966 81.371                                     81.970    26.     0.807      96.773 84.289                                     84.730    28.     0.869      97.642 87.761                                     87.490    26.     0.807      98.449 91.310                                     90.250    22.     0.683      99.131 94.606                                     93.010    10.     0.310      99.442 96.246                                     95.770    3.      0.093      99.535 96.783                                     98.530    5.      0.155      99.690 97.757                                     101.290   3.      0.093      99.783 98.393                                     104.050   7.      0.217      100.000                                                                               100.000                                    ______________________________________                                          DN = 45./2                                                                     A = 60.81                                                                      D50 = 45.27                                                                    K = 4                                                                          A(K) = 0.1247                                                                  FQS = 1.7246                                                                   DN/D50 = 1.0100                                                                DA/D50 = 1.3434                                                                DA/DN = 1.3301                                                           

These data are illustrated graphically in FIGS. 5 to 10.

Example 3

The procedure described in Example 1 is employed, the following amount being used:

0.5005 kg/h of SiCl₄ with 0.1 Nm³ /h of nitrogen as carrier gas, air 0.726 Nm³ /h, oxygen 0.152 Nm³ /h, (core) hydrogen 0.14 Nm³ /h.

0.05 Nm³ /h of hydrogen are fed into the annular nozzle surrounding the mouth of the burner. The gas mixture flows at a rate of 12 m/sec (referred to standard conditions) and at a temperature of 124° C. from the nozzle opening of the burner.

0.225 kg/h of trimethyl borate are vaporized in a separate vaporization device and are likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm³ /h).

The specific surface of the boron oxide-silicon dioxide mixed oxide is 39 m² /g, and the analytically determined B₂ 0₃ content is 23.0 wt. % (=7.1 wt. % of boron, calculated as boron).

The adjustment parameters are again summarized in Table 1.

Example 4

The procedure described in Example 1 is adopted, the following amounts being used:

0.365 kg/h of SiCl₄, with 0.1 Nm³ /h of nitrogen as carrier gas, air 0.759 Nm³ /h, oxygen 0.159 Nm³ /h, hydrogen 0.103 Nm³ /h.

0.05 Nm³ /h of hydrogen are fed into the annular nozzle surrounding the mouth of the burner.

0.2575 kg/h of trimethyl borate are vaporized in a separate vaporization device and are likewise fed into the mixing chamber of the burner by entrainment in a stream of nitrogen (0.02 Nm³ /h).

The gas mixture flows at a rate of 12 m/sec (referred to standard conditions) and at a temperature of 125° C. from the nozzle opening of the burner.

The specific surface of the boron oxide-silicon dioxide mixed content is 30 m² /g, and the analytically determined B₂ O₃ content is 29.0 wt. % (=9.0 wt. % or boron calculated as boron).

The adjustment parameters are again summarized in Table

                                      TABLE 1                                      __________________________________________________________________________     Experimental conditions and flame parameters calculated therefrom in           the preparation of pyrogenic boron oxide-silicon dioxide mixed oxides.            SiCl.sub.4                                                                         TMB Air O.sub.2                                                                            H.sub.2 core                                                                       N.sub.2 core                                                                       gamma                                                                              lambda                                                                             BET B.sub.2 O.sub.3                         No.                                                                               [kg/h]                                                                             [kg/h]                                                                             [Nm.sup.3 /h]                                                                      [Nm.sup.3 /h]                                                                      [Nm.sup.3 /h]                                                                      [Nm.sup.3 /h]                                                                      [-] [-] [m.sup.2 /g]                                                                       [wt. %]                                 __________________________________________________________________________     1  1.0796                                                                             0.0643                                                                             0.510                                                                              0.107                                                                              0.300                                                                              0.12                                                                               1.27                                                                               1.01                                                                               45   4.2                                    2  0.9387                                                                             0.1777                                                                             0.760                                                                              0.159                                                                              0.268                                                                              0.12                                                                               1.78                                                                               1.03                                                                               34  13.3                                    3  0.5005                                                                             0.2250                                                                             0.726                                                                              0.152                                                                              0.140                                                                              0.12                                                                               2.72                                                                               1.04                                                                               39  23.0                                    4  0.3651                                                                             0.2575                                                                             0.759                                                                              0.159                                                                              0.103                                                                              0.12                                                                               3.67                                                                               1.04                                                                               30  29.0                                    __________________________________________________________________________      TMB = Trimethyl borate B(OCH.sub.3).sub.3                                      Ratio H.sub.2 gamma = Ratio of fedin hydrogen in the core (taking into         account the hydrogen contained in TMB) to stoichiometrically required          hydrogen                                                                       Ratio O.sub.2 lambda = Ratio of fedin oxygen (atmospheric oxygen +             additionally added O.sub.2) in the burner to stoichiometrically required       oxygen                                                                   

Deacidification and Removal of Raw Material Residues

In order to remove raw material residues that have possibly incompletely reacted and are still adhering to the product, and to reduce the chloride content of the samples, the oxides prepared according to Examples 1 to 9 can undergo a further temperature treatment stage.

For this purpose the powders are treated with moist air in a countercurrent downpipe arrangement at temperatures between 400° and 700° C. (preferably 650° C.). (Deacidification)

The analysis date of the powders prepared according to Examples 1 to 4 before and after deacidification are summarised in Tables 2 and 3.

                                      TABLE 2                                      __________________________________________________________________________     Analysis data of the samples obtained according to Examples 1 to 4             (B--Si-mixed oxide)                                                            Before deacidification                                                                               After deacidification                                                      Cl                 Cl                                           BET                                                                               B.sub.2 O.sub.3                                                                    TV  GV  Content                                                                            BET                                                                               B.sub.2 O.sub.3                                                                    TV  GV  Content                                   No m.sup.2 /g                                                                        Wt. %                                                                              Wt. %                                                                              Wt. %                                                                              Wt. %                                                                              m.sup.2 /g                                                                        Wt. %                                                                              Wt. %                                                                              Wt. %                                                                              ppm                                       __________________________________________________________________________     1  45  4.2                                                                               1.5 0.4 200 46  3.6                                                                               0.6 0.1 138                                       2  34 13.3                                                                               1.8 2.7 142 37 10.1                                                                               0.4 1.5 87                                        3  39 23.0                                                                               0.8 3.1 87  44 22.2                                                                               0.5 2.5 18                                        4  30 29.0                                                                               1.0 3.8 48  33 28.6                                                                               0.7 1.3 13                                        __________________________________________________________________________      TV = Drying loss (2 h at 10° C., according to DIN/ISO 787/II, ASTM      D 280, JIS K 5101/21)                                                          GV = Annealing loss (2 h at 1000° C., according to DIN 55921, ASTM      D 1208, JIS K 5101/23, referred to the substance dried for 2 hours at          105° C.                                                           

                  TABLE 3                                                          ______________________________________                                         Further analysis data of the samples obtained according to Examples 1          to 4 (B--Si-mixed oxide)                                                       Before deacidification                                                                            After deacidification                                                          Bulk  Tamped          Bulk  Tamped                               BET           density                                                                              density                                                                              BET       density                                                                              density                         No.  m.sup.2 g                                                                             pH     g/l   g/l   m.sup.2 /g                                                                          pH   g/l   g/l                             ______________________________________                                         1    45     3.84   58    69    46   4.09 88    113                             2    34     3.89   46    52    37   3.93 91    115                             3    39     3.69   39    46    44   3.88 69    87                              4    30     3.52   41    48    33   3.88 94    118                             ______________________________________                                          pH = pH in 4 t aqueous suspension.,                                            Tamped density according to DIN/ISO 787/XI, JIS K 5101/18 (not screened).

Examples of the Preparation of Boron Oxide-Aluminum Oxide-Silicon Dioxide Mixed Oxide

The burner arrangement for Examples 5 to 9 is illustrated diagrammatically in FIG. 2.

Example 5

The two liquids SiCl₄ and trimethyl borate are mixed In the desired ratio before being vaporised and are then converted together into the gaseous phase in a vaporiser at 135° C. 0.876 kg/h of SiCl₄ is vaporised together with 0.231 kg/h of trimethyl borate. The gases are conveyed with a stream of nitrogen at a rate of 0.120 Nm³ /h into the mixing chamber of the burner mentioned in Example 1.

The temperature of the furnace in which the burner is housed is ca. 240° C. 0.056 kg/h of AlCl₃ is vaporized in a separate vaporiser at temperatures of ca. 230° C. and is similarly conveyed in a stream of nitrogen as carrier gas at a rate of 0.120 Nm³ /h into the mixing chamber of this burner.

0.938 Nm3/h of air as well as 0.196 Nm³ /h of oxygen and 0.254 Nm³ /h of hydrogen are introduced into the burner mixing chamber.

The gas mixture flows at a rate of 17 m/sec (referred to standard conditions) from the nozzle opening of the burner and bums in a reaction chamber (flame pipe).

The temperature of the gas mixture (SiC₄, AlCl₃, air, hydrogen, oxygen, and trimethyl borate) is measured at the mouth of the burner and found to be 235° C. 0.05 Nm³ /h of hydrogen (jacket hydrogen, room temperature) is fed into the annular nozzle surrounding the mouth of the burner.

After flame hydrolysis the reaction gases and the boron oxide-silicon dioxide-aluminium oxide mixed oxide are sucked through a cooling system by applying a vacuum and cooled to ca. 100° to 160° C. The solid is separated from the waste gas stream in a filter. The solid (boron oxide aluminium oxide-silicon dioxide mixed oxide) is in the form of a white, finely divided powder.

The specific surface of the boron oxide-aluminum oxide silicon dioxide mixed oxide is 28 m² /g, the analytically determined B₂ 0₃ content is 12.3 wt. % (≅3.8 wt. % of boron) and the analytically determined Al₂ 0₃ content is 3.9 wt. %.

Example 6

The procedure described in Example 5 is adopted, the following amounts being used: 0.83 kg/h of SiCl₄ is vaporised together with 0.219 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a rate of 0.120 Nm³ /h into the mixing chamber of the burner mentioned in Example 1. 0.108 kg/h of AlCl₃ is vaporised in a separate vaporiser at a temperature of ca. 240° C. and conveyed in a stream of nitrogen as carrier gas at a rate of 0.120 Nm³ /h into the mixing chamber of this burner.

0.782 Nm³ /h of air as well as 0.164 Nm³ /h of oxygen and 0.255 Nm³ /h of hydrogen are introduced into the same mixing chamber.

The gas outflow temperature at the mouth or the burner is 235° C.

After flame hydrolysis and oxidation, and separation of the gaseous reaction gases, a boron oxide-aluminium oxide silicon dioxide mixed oxide is obtained having a BET surface of 30 m² /g, the analytically determined B₂ 0₃ content being 13.9 wt. % (≅4.3 wt. % of boron) and the analytically determined Al₂ 0₃ content being 8.3 wt. %.

Example 7

The procedure described in Example 5 is adopted, the following amounts being used:

0.788 kg/h of SiCl4 is vaporised together with 0.208 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a i rate of 0.120 Nm³ /h into the mixing chamber of the burner mentioned in Example 1. 0.151 kg/h of AlCl₃ is vaporised in a separate vaporiser at temperatures of ca. 240° C. and conveyed in a stream of carrier gas at a rate of 0.120 Nm³ /h into the mixing chamber of this burner. 0.762 Nm3/h of air as well as 0.160 Nm³ /h of oxygen and 0.257 Nm³ /h of hydrogen are introduced into the same mixing chamber.

The gas outlet temperature at the mouth of the burner is 235° C.

After flame hydrolysis and separation ot the gaseous reaction gases, a boron oxide-aluminium oxide-silicon dioxide mixed oxide is obtained having a BET surface of 37 m² /g, the analytically determined B₂ 0₃ content being 14.3 wt. % (≅4.4 wt. % of boron) and the analytically determined Al₂ 0₃ content being 12.9 wt. %.

Example 8

The procedure described in Example 5 is adopted, the following amounts being used:

0.721 kg/h of SiCl4 is vaporised together with 0.190 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a rate of 0.120 Nm³ /h into the mixing chamber of the burner mentioned in Example 1. 0.232 kg/h of AlCl₃ is vaporised in a separate vaporiser at a temperature of ca. 240° C. and conveyed in a stream of carrier gas at a rate of 0.120 Nm³ /h into the mixing chamber of this burner.

0.842 Nm³ /h of air as well as 0.176 Nm³ /h of oxygen and 0.259 Nm³ /h of hydrogen are introduced into the same mixing chamber.

The gas outlet temperature at the mouth of the burner is 235° C.

After flame hydrolysis and separation of the gaseous reaction gases, a boron oxide-aluminium oxide-silicon 29 m² /g, the analytically determined B₂ 0₃ content being 14.6 wt. % (≅4.5 wt. % of boron) and the analytically determined Al₂ 0₃ content being 19.3 wt. %.

Example 9

The procedure described in Example 5 is adopted, the following amounts being used:

0.641 kg/h of SiCl₄ is vaporised together with 0.169 kg/h of trimethyl borate and conveyed in a stream of nitrogen at a rate of 0.120 Nm³ /h into the mixing chamber of the burner mentioned in Example 5. 0.321 kg/h of AlCl³ is vaporised in a separate vaporiser at a temperature of ca. 240° C. and conveyed in a stream of carrier gas at a rate of 0.120 Nm³ /h into the mixing chamber of this burner.

0.800 Nm³ /h of air as well as 0.167 Nm³ /h of oxygen and 0.261 Nm³ /h of hydrogen are introduced into the same mixing chamber.

The gas outlet temperature at the mouth of the burner is 235° C.

After flame hydrolysis and separation of the gaseous reaction gases a boron oxide-aluminium oxide-silicon dioxide mixed oxide is obtained having a BET surface of 37 m² /g, the analytically determined B₂ 0₃ content being 12.9 wt. % (≅4.0 wt. % of boron) and the analytically determined Al₂ 0₃ content being 29.0 wt. %.

The boron oxide-aluminium oxide-silicon dioxide mixed oxide obtained has the following particle size distribution:

    ______________________________________                                         Total number of particles                                                                            (N) 4349                                                 Particle diameter, arithmetic mean                                                                   (DN) 77.66 (NM)                                          Particle diameter, averaged over surface                                                             (DA) 101.81 (NM)                                         Percentage distribution                                                        ______________________________________                                    

    ______________________________________                                                                      Sum   Sum                                         Diameter  Number  Number     Number                                                                               Wt. %                                       D (NM)    N       %          %     %                                           ______________________________________                                         14.900    367.    8.439      8.439 0.038                                       20.420    90.     2.069      10.508                                                                               0.062                                       25.940    53.     1.219      11.727                                                                               0.092                                       31.460    47.     1.081      12.808                                                                               0.138                                       36.980    54.     1.242      14.049                                                                               0.224                                       42.500    95.     2.184      16.234                                                                               0.454                                       48.020    143.    3.288      19.522                                                                               0.953                                       53.540    198.    4.553      24.074                                                                               1.912                                       59.060    214.    4.921      28.995                                                                               3.302                                       64.580    255.    5.863      34.859                                                                               5.468                                       70.100    293.    6.737      41.596                                                                               8.650                                       75.620    315.    7.243      48.839                                                                               12.946                                      81.140    297.    6.829      55.668                                                                               17.949                                      86.660    306.    7.036      62.704                                                                               24.230                                      92.180    300.    6.898      69.602                                                                               31.640                                      97.700    278.    6.392      75.994                                                                               39.815                                      103.220   248.    5.702      81.697                                                                               48.416                                      108.740   176.    4.047      85.744                                                                               55.553                                      114.260   123.    2.828      88.572                                                                               61.339                                      119.780   129.    2.966      91.538                                                                               68.330                                      125.300   84.     1.931      93.470                                                                               73.541                                      130.820   86.     1.977      95.447                                                                               79.613                                      136.340   64.     1.472      96.919                                                                               84.728                                      141.860   42.     0.966      97.885                                                                               88.509                                      147.380   29.     0.667      98.551                                                                               91.436                                      152.900   25.     0.575      99.126                                                                               94.254                                      158.420   15.     0.345      99.471                                                                               96.135                                      163.940   8.      0.184      99.655                                                                               97.247                                      169.460   4.      0.092      99.747                                                                               97.861                                      174.980   4.      0.092      99.839                                                                               98.536                                      180.500   3.      0.069      99.908                                                                               99.093                                      186.020   1.      0.023      99.931                                                                               99.296                                      191.540   1.      0.023      99.954                                                                               99.517                                      197.060   2.      0.046      100.000                                                                              100.000                                     ______________________________________                                          DN = 77/66                                                                     DA = 101.81                                                                    D50 = 76.56                                                                    K = 6                                                                          A(K) = O.0991                                                                  FQS = 1.7952                                                                   DN/D50 = 1.U144                                                                DA/D50 = 1.3298                                                                DA/DN = 1.3110                                                           

These data are shown graphically in FIGS. 11 to 16.

                                      TABLE 4                                      __________________________________________________________________________     Experimental conditions and flame parameters calculated therefrom in           the preparation of pyrogenic boron-silicon-aluminum mixed oxides                  SiCl4                                                                              TMB AlC.sub.3                                                                          Air O.sub.2                                                                            H.sub.2                                                                            gamma                                                                              lambda                                                                             BET                                         No [kg/h]                                                                             [kg/h]                                                                             [kg/h]                                                                             [Nm.sup.3 /h]                                                                      [Nm.sup.3 /h]                                                                      [Nm.sup.3 /h]                                                                      [-] [-] [m.sup.2 /g]                                __________________________________________________________________________     5  0.876                                                                              0.231                                                                              0.056                                                                              0.938                                                                              0.196                                                                              0.254                                                                              1.95                                                                               1.10                                                                               28                                          6  0.830                                                                              0.219                                                                              0.108                                                                              0.782                                                                              0.164                                                                              0.255                                                                              1.90                                                                               0.97                                                                               30                                          7  6.788                                                                              0.208                                                                              0.151                                                                              0.762                                                                              0.160                                                                              0.257                                                                              1.87                                                                               0.97                                                                               27                                          8  0.721                                                                              0.190                                                                              0.232                                                                              0.842                                                                              0.176                                                                              0.259                                                                              1.78                                                                               1.10                                                                               29                                          9  0.641                                                                              0.169                                                                              0.321                                                                              0.800                                                                              0.167                                                                              0.261                                                                              1.70                                                                               1.11                                                                               29                                          __________________________________________________________________________      TMB = Trimethyl borate B(OCH.sub.3).sub.3                                      Ratio H.sub.2 gamma = Ratio of fedin hydrogen in the core (taking into         account the hydrogen contained in TMB) to stoichiometrically required          hydrogen                                                                       Ratio O.sub.2 lambda = Ratio of fedin oxygen (atmospheric oxygen +             additionally added O.sub.2) in the  burner to stoichiometrically required      oxygen                                                                   

                                      TABLE 5                                      __________________________________________________________________________     Analytical data of the samples (Al--B--Si mixed oxide) obtained                according to Examples 5 to 9                                                   Before deacidification                                                                               After deacidification                                       BET                                                                               B.sub.2 O.sub.3                                                                    Al.sub.2 O.sub.3                                                                   TV  GV  BET                                                                               B.sub.2 O.sub.3                                                                    Al.sub.2 O.sub.3                                                                   TV  GV                                           m.sup.2 /g                                                                        Wt. %                                                                              Wt. %                                                                              Wt. %                                                                              Wt. %                                                                              m.sup.2 /g                                                                        Wt. %                                                                              Wt. %                                                                              Wt. %                                                                              Wt. %                                     __________________________________________________________________________     5  28 12.3                                                                                3.9                                                                               0.5 3.2 27 13.5                                                                                4.5                                                                               0.2 2.1                                       6  30 13.9                                                                                8.3                                                                               1.6 3.3 29 13.9                                                                                9.3                                                                               0.2 2.2                                       7  27 14.3                                                                               12.9                                                                               0.7 3.5 29 13.0                                                                               12.9                                                                               0.6 1.7                                       9  27 12.9                                                                               29.0                                                                               0.6 1.9 34 12.4                                                                               27.0                                                                               0.3 1.5                                       __________________________________________________________________________      TV = Drying loss (2 h at 105° C., according to DIN/ISO 787/II, AST      D 280, JIS K 5101/21)                                                          GV = Annealing loss (2 h at 1000° C., according to DIN 5bY21, ASTM      D 1208, JIS K 5101/23, referred to the substance dried for 2 hours at          105° C.                                                           

                                      TABLE 6                                      __________________________________________________________________________     Further analytical data of the samples (Al--B--Si-mixed oxide)                 obtained according to Examples 5 to 9                                          Before deacidification                                                                              After deacidification                                           Cl     Bulk                                                                               Tamped Cl     Bulk                                                                               Tamped                                         BET                                                                               content                                                                               density                                                                            density                                                                               content                                                                               density                                                                            density                                     No m.sup.2 /g                                                                        ppm pH g/l g/l m.sup.2 g                                                                         ppm pH g/l g/l                                         __________________________________________________________________________     5  28 150 3.47                                                                              65  82  27 130 3.90                                                                              175 236                                         6  30 148 3.79                                                                              76  91  29  85 4.06                                                                              190 240                                         7  27 132 3.84                                                                              80  97  29 115 4.38                                                                              210 270                                         8  29 223 4.21                                                                              74  97  32 310 4.45                                                                              202 255                                         9  27 210 4.33                                                                              75  90  34 414 4.55                                                                              215 276                                         __________________________________________________________________________      pH = pH in 4 percent aqueous suspension.                                       Tamped density according to DIN/ISO 787/XI, JIS K 5101/18 (not screened). 

What is claimed is:
 1. A boron oxide-silicon dioxide mixed oxide in powdered form, for making glass, having a BET surface of 30-100 m² /g and a B₂ O₃ content of from 10.1 to 40 wt. %.
 2. A boron oxide-silicon dioxide mixed oxide according to claim 1, containing at least one further oxide of a metal or a metalloid.
 3. A boron oxide-silicon dioxide mixed oxide according to claim 1, wherein said BET surface is in the range of 30 to 80 m² /g.
 4. A boron oxide-silicon dioxide mixed oxide according to claim 3, containing at least one further oxide of a metal or a metalloid.
 5. A boron oxide-silicon dioxide mixed oxide according to claim 1, wherein said BET surface is in the range of 30 to 50 m² /g.
 6. A boron oxide-silicon dioxide mixed oxide according to claim 5, containing at least one further oxide of a metal or a metalloid. 